It is generally known in the spinning industry that, apart from the irregularities in the yarn, which have the most varied amplitudes, periodic deviations occur which are later clearly visible in the yarn (yarn conversion table) and also in the woven fabric as a result of their position side by side. This phenomenon is generally known as the moire effect. The causes of such periodic faults in the yarn originate from the processing, during which disturbances occur periodically. In a ring spinning machine, for example, such disturbances may result from drawing rollers oscillating. In rotor spinning, such disturbances appear particularly strikingly as a result of deposits in the rotor. Because it is known what generally causes the periodic disturbances, conclusions regarding the source of disturbance can be drawn from a yarn unevenness diagram.
In open-end spinning, with regard to moire detection, it is known that the visible spacing of thick places appearing on the yarn appear according to the circumference of the rotor. The difficulty of detection, however, lies in the fact that the delivery speed is variable and so the moire frequency is likewise variable.
It is true that it is possible to check a yarn production afterwards by means of random samples with the aid of a spectograph (Uster), for whether such a moire effect has occurred or not; nevertheless, serious damage results because the production has already taken place, and, in any case, a relatively large amount of faulty yarn may have been produced depending on the frequency of the checking.
It is true that it is already known to detect the point of the disturbance directly and immediately by monitoring individual spindles so as to avoid a large faulty production (DT-OS 2.409.882 corresponding to U.S. Pat. No. 4,007,457 ). According to the earlier proposal, this is effected in that an electric signal is produced by measuring the yarn cross-section or yarn diameter, which signal is subjected to an evaluation by means of at least one non-linear correction member.
In order to detect periodic faults in the spun yarn, it is necessary, in this case, to convey the signal originating from a monitored yarn through electric filters which are adjusted to the expected repetition frequency of these faults, hereinafter termed moire frequency, and a detector has to be present at each spinning station. Since the delivery speed of the spun thread is variable, either narrow-band filters have to be used, which are variable in their midband frequency, which leads to considerable costs, or very wide-band filters have to be used so that the moire frequency is transmitted thereby even with different yarn-delivery speeds. It is true that in the latter case, the costs for the circuits are reduced somewhat, but the wide-band nature of the filters means that a considerable proportion of the frequencies of the normal irregularity in the spun yarn can pass. A moire effect is therefore only detected, when using wide-band filters, if it stands out very distinctly from the normal irregularity of the thread.